[6.07] Blue Straggler Binaries and Mass Transfer

We have monitored the radial velocities of ten candidate
field blue stragglers for time spans ranging from 15.9 to
19.0 years, or until a binary orbital solution was obtained.
Three of the candidate blue stragglers show no signs of
velocity variation, one appears to be a very long-period
binary, and the remaining six are single-lined spectroscopic
binaries, with periods ranging from 167 to 844 days. The
orbital eccentricities are all low, e < 0.30, and =
0.11. Five of the six binary orbits have very low
eccentricities, with = 0.07. We have re-analyzed the
velocity data from Preston & Sneden (2000) and have derived
orbital solutions similar to theirs for ten of the
spectroscopic binaries among their ``blue metal-poor'' stars
with [Fe/H]~\leq -0.6. Our orbital solutions for their
seven binaries with periods longer than 20 days all have low
eccentricities, with e \leq 0.26 and = 0.11. Of the
five binary stars in our program with published abundances
of lithium, all have been found to be deficient. In
contrast, two of the three apparently single stars have
published lithium abundances and show no deficiency. The
mass functions for the thirteen binaries in our program and
that of Preston & Sneden (2000) are consistent with the
unseen companions all being white dwarfs with M \approx
0.55~M\odot and random orbital inclinations. {\em Our
results are consistent with all field blue stragglers being
binary systems with long periods and low eccentricities, the
primary stars being deficient in lithium and the secondary
stars being normal-mass white dwarfs, indicating mass
transfer is the cause of the phenomenon.}